fix(client): suppress unhandled rejection from WithStartWorkflowOperation

[mjameswh]

Summary

WithStartWorkflowOperation creates workflowHandlePromise eagerly with no .catch() handler. When executeUpdateWithStart fails at the gRPC transport level before any response is received, onStartError rejects workflowHandlePromise and re-throws to the caller. The caller's promise is rejected correctly, but the rejected workflowHandlePromise has no handler, causing a spurious unhandledRejection event that terminates the process.

Fix: add a no-op .catch(() => undefined) on workflowHandlePromise after construction — the same pattern used in packages/worker/src/activity.ts and packages/worker/src/worker.ts. Callers who await startOp.workflowHandle() still observe the rejection normally; if the caller ignores executeUpdateWithStart entirely, that unhandled rejection is still reported.

Also adds a regression test that verifies no spurious unhandledRejection event fires when the failure occurs and the caller never awaits workflowHandle().

Fixes #1960